a) Field of the Invention
The present invention relates to a crystal growth method and apparatus, and more particularly to a crystal growth method and apparatus suitable for crystal growth of a substance having a high vapor pressure near at a melting point, such as a compound semiconductor.
b) Description of the Related Art
Known crystal growth methods include a melted liquid growth (melt growth) method, a solution growth method, a vapor phase growth method and the like. With the melted liquid growth method, a substance for crystal growth is heated to a melting point or higher to form melted liquid, and a seed crystal is brought into contact with the melted liquid or a region having a temperature lower than the melting point is formed, to thus grow a crystal.
With the a solution growth method, substance for crystal growth is dissolved in solvent to form solution, a solubility is lowered through evaporation of the solvent, the temperature falls or the like to form a supersaturated region, and crystal is grown on a seed crystal or the like.
With the vapor phase growth method, a substance for crystal growth or its source material is transported to grow a crystal on a seed crystal or the like. Each of the crystal growth methods has specific characteristics which will be described by taking as an example the crystal growth of a compound semiconductor.
A compound semiconductor of group III-V, II-VI or the like has generally a high vapor pressure near the a melting point or higher. The melted liquid crystal growth method requires a growth container resistant to a high pressure because a vapor pressure above the melted liquid becomes high.
The solution crystal growth method does not require heating to the melting point or higher, and can grow a crystal if a solution is prepared. For example, a vertical container is used as a growth container, and a temperature gradient is formed between the upper and lower regions of the container. A source material is placed in the upper high temperature region of the solution and a seed crystal is placed in the lower low temperature region to grow a single crystal on the seed crystal.
With reference to FIGS. 3A and 3B, a solution crystal growth apparatus and method disclosed in Japanese Patent Laid-open Publication No. 8-208365 filed by the present assignee will be described. This solution crystal growth method grows a bulk single crystal having a larger diameter by using a seed crystal having a smaller diameter.
This method will be detailed by taking as an example a crystal growth of a group II-VI compound semiconductor ZnSe by using a Se--Te solvent. ZnSe is a promising substance for blue light emitting semiconductor elements.
FIG. 3A shows a crystal growth apparatus. The right side in FIG. 3A is a cross sectional view of the crystal growth apparatus and the left side is a graph showing a temperature distribution set in a furnace. A crystal growth container 1 is prepared which is made of a quartz tube having a proper diameter. The surface of the crystal growth container 1 is etched by hydrofluoric acid to clean it.
A heat sink 7 made of a material having a good thermal conductivity such as carbon is placed on the bottom of the crystal growth container 1 with the cleaned surface. After vacuum baking is performed, the heat sink 7 is fixed by shrinking the inner diameter of the crystal growth container 1 or by other means.
As a seed crystal 2, a ZnSe single crystal having the plane (1 1 1) is prepared. A ZnSe single crystal having a plane different from the plane (1 1 1) may be used. After the seed crystal 2 is polished to have a mirror surface, it is washed and etched to have a mirror surface. The prepared seed crystal 2 is placed on the central upper surface of the heat sink 7.
It is preferable to form a recess in the upper surface layer of the heat sink 7 for receiving or accomodating the seed crystal 2 in this recess. In this case, it is also preferable to set the level of the upper surface of the seed crystal 2 higher than the upper surface of the heat sink 7.
In order to fix the seed crystal 2, a seed crystal fixer 3 made of quartz and having a ring shape is placed on the seed crystal 2 and partially fixed to the crystal growth container 1. The ring-shaped seed crystal fixer 3 has a tapered inner surface. The inner diameter at the smaller diameter end of the tapered inner surface is smaller than the diameter of the seed crystal 2, and the inner diameter at the larger diameter end is roughly equal to the inner diameter of the crystal growth container 1.
As shown in FIG. 3A, the ring-shaped seed crystal fixer 3 has a flat bottom surface and a conically tapered upper surface, so that the peripheral region of the fixer 3 is thick and the central region is thin. An angle between the bottom surface and the tapered upper surface is set in a range from an angle of 60.degree. to an angle smaller than 90.degree. or preferably in a range from an angle of 70.degree. to an angle smaller than 80.degree..
The fixer 3 has a generally flat bottom surface and is formed with a circular step in the central region thereof in order to reliably fix the seed crystal 2. The diameter of the step is generally equal to that of the seed crystal 2 and the height thereof is lower than a height of the seed crystal 2 projecting out of the heat sink 7.
A projection 5 for preventing a fall of a source material 4 is formed on the inner wall of the crystal growth container 1 at a proper distance from the upper surface of the heat sink 7.
Thereafter, a Se--Te mixture having a predetermined composition as a solvent 6 and ZnSe polycrystal as a source material (crystal) 4 are loaded in the crystal growth container 1. The quantity of the solvent 6 is set so that it completely covers the seed crystal 2 and source material 4.
The source material 4 is supported by the projection or source material support jig 5. A distance between the seed crystal 2 and source material 4 is preferably 20 mm to 80 mm or it is optimum to set to 40 mm to 60 mm. The source material 4 is disposed facing the seed crystal 2 and has preferably a disk shape so as to facilitate the support thereof.
The crystal growth container 1 shown in FIG. 3A has the projection for preventing the fall of the source material 4. The fall of the source material 4 may be prevented by other means. For example, a quartz tube having a diameter larger than that of the quartz tube where the solvent is filled, is connected to the latter quartz tube, and a source material of a disk shape having a diameter nearly the same as that of the former quartz tube is used. A step between the two quartz tubes can prevent the fall of the source material.
The crystal growth container 1 accommodating the source material 4, solvent 6 and seed crystal 2 in the above manner is connected to a vacuum pump to evacuate the inside of the container 1 to a vacuum degree higher than 2.times.10.sup.-6 Torr, and thereafter the open end of the container 1 is hermetically sealed.
The crystal growth container 1 is turned laterally or upside down to dissolve (melt) the solvent. The seed crystal 2 does not contact the solvent, and only the source material 4 contacts the solvent. In this state, the solvent is saturated by the solute to form a saturated solution. A saturated solution is prepared at a growth temperature by setting the solvent to this growth temperature. Thereafter, the crystal growth container is turned upright. Since the solvent is a saturated solution, dissolution of the seed crystal can be suppressed to a very low level.
The crystal growth container 1 thus prepared is placed in an electric furnace having a temperature distribution shown in the left side in FIG. 3A.
The source material 4 in the high temperature area is dissolved in the solvent 6 up to a saturation solubility in the high temperature area. Source material components dissolved in the solvent 6 move also to the low temperature area through diffusion so that the solution in the low temperature area has a supersaturated state.
As the seed crystal 2 comes in contact with the supersaturated solution, a bulk single crystal is grown on the seed crystal. In this case, the crystal is grown along the tapered surface of the seed crystal fixer 3 and along the inner wall of the crystal growth container 1, and the grown crystal gradually increases in diameter. The grown crystal having a diameter larger than that of the seed crystal 2 can be obtained.
FIG. 3B is a cross sectional view showing another example of a crystal growth container 1a. A quartz tube 1c having a smaller diameter and a quartz tube 1e having a larger diameter are connected by a horn-shaped quartz tube 1d having one cut end with a diameter equal to that of the quartz tube 1c and the other cut end with a diameter equal to that of the quartz tube 1e. An angle between the slanted surface of the horn-shaped quartz tube 1d and a plane perpendicular to the center axis of the tube 1d is preferably in a range from an angle of 60.degree. to an angle smaller than 90.degree., or it is optimum to set it in a range from an angle of 70.degree. to an angle smaller than 80.degree..
The quartz tube 1c and the smaller diameter cut end of the quartz tube 1d, as well as the quartz tube 1e and the larger diameter cut end of the quartz tube 1d, are smoothly coupled to form the crystal growth container 1a. A projection 5 for preventing the fall of a source material is formed on the inner wall of the crystal quartz container 1e at a proper height position.
A heat sink 7 is housed in the smaller diameter region of the crystal growth container 1a, and a seed crystal 2 is placed on the upper surface of the heat sink 7. The seed crystal 2 is a ZnSe single crystal which is the same as that used with the container 1 shown in FIG. 3A, and has a diameter nearly equal to the inner diameter of the quartz tube 1c.
Next, a horn-shaped seed crystal fixer 3a is inserted into and fixed to the quartz tube 1d to thereby fix the seed crystal 2, the fixer 3a having the outer surface conformal to the inner surface of the quartz tube 1d. Similar to the inner surface of the quartz tube 1d, the inner surface of the seed crystal fixer 3a has an inclination angle of 60.degree. to 90.degree., or preferably 70.degree. to 80.degree., relative to the plane perpendicular to the center axis of the quartz tube 1d.
Similar to the container 1 shown in FIG. 3A, solvent 6 and a source material 4 are loaded in the container 1a which is then evacuated and hermetically sealed for crystal growth. In this case, a crystal is grown along the inner surfaces of the seed crystal fixer 3a and quartz tubes 1d and 1e, so that a crystal having a diameter larger than that of the seed crystal 2 can be obtained.
The solution crystal growth can lower the growth temperature. However, if a vapor pressure of solvent is high, the high temperature crystal growth becomes difficult. As the growth temperature lowers, the growth speed lowers.
After the crystal growth, the temperature of the solution is lowered and the crystal growth container is cracked or destroyed to pick up the grown crystal. In most cases, the used solution is dumped so that material cost rises.
Although the diameter of the grown crystal becomes large by growing the crystal having a tapered region along the growth direction, nuclei are likely to be generated in the tapered region so that it is not easy to grow a single crystal with good reproductivity.